Photocatalytic properties of graphene-supported titania clusters from density-functional theory

Density-functional theory calculations of (TiO₂)_n clusters (n = 1–5) in the gas phase and adsorbed on pristine graphene as well as graphene quantum dots are presented. The cluster adsorption is found to be dominated by van der Waals forces. The electronic structure and in particular the excitation energies of the bare clusters and the TiO₂/graphene composites are found to vary largely in dependence on the size of the respective constituents. This holds in particular for the energy and the spatial localization of the highest occupied and lowest unoccupied molecular orbitals. In addition to a substantial gap narrowing, a pronounced separation of photoexcited electrons and holes is predicted in some instances. This is expected to prolong the lifetime of photoexcited carriers. Altogether, TiO₂/graphene composites are predicted to be promising photocatalysts with improved electronic and photocatalytic properties compared to bulk TiO₂.     

Density Functional Theory Study of Optical and Electronic Properties of (TiO2)n=5,8,68 Clusters for Application in Solar Cells

A range of solution-processed organic and hybrid organic−inorganic solar cells, such as dye-sensitized and bulk heterojunction organic solar cells have been intensely developed recently. TiO₂ is widely employed as electron transporting material in nanostructured TiO₂ perovskite-sensitized solar cells and semiconductor in dye-sensitized solar cells. Understanding the optical and electronic mechanisms that govern charge separation, transport and recombination in these devices will enhance their current conversion efficiencies under illumination to sunlight. In this work, density functional theory with Perdew-Burke Ernzerhof (PBE) functional approach was used to explore the optical and electronic properties of three modeled TiO₂ brookite clusters, (TiO₂)_n=5,8,68. The simulated optical absorption spectra for (TiO₂)₅ and (TiO₂)₈ clusters show excitation around 200–400 nm, with (TiO₂)₈ cluster showing higher absorbance than the corresponding (TiO₂)₅ cluster. The density of states and the projected density of states of the clusters were computed using Grid-base Projector Augmented Wave (GPAW) and PBE exchange correlation functional in a bid to further understand their electronic structure. The density of states spectra reveal surface valence and conduction bands separated by a band gap of 1.10, 2.31, and 1.37 eV for (TiO₂)₅, (TiO₂)₈, and (TiO₂)₆₈ clusters, respectively. Adsorption of croconate dyes onto the cluster shifted the absorption peaks to higher wavelengths.     

Effect of the Structure of Small Anatase Nanoparticles on the Localization of Photogenerated Charge Carriers

The structure of nanoparticles and the spatial arrangement of photogenerated thermalized charge carriers are studied for a series of isomers of small anatase nanoparticles (TiO₂)₂₉(H₂O)₄, (TiO₂)₇₀(H₂O)₄, and (TiO₂)₇₀ with faces (001) and (101) on the surface. It is shown that the location of surface hydroxyl groups and their replacement by surface oxygen atoms affect the nature and degree of deformation of the nanoparticle structure. The location of the boundary orbitals depends both on the size of the nanoparticles and on the location of the hydroxyl groups, as well as on the degree of dehydroxylation, which leads to the replacement of the hydroxyl groups by the surface oxygen atoms. In the case of a certain arrangement of hydroxyl groups or surface oxygen atoms, uncharged small stoichiometric anatase nanoparticles begin to absorb light in the visible region of the spectrum (the band gap width E_g decreasing to 2.25 eV). This is associated with the energy levels at the edge of the band gap near the valence band and the conduction band.     

Characterization of the Excited State on Methanol/TiO2(110) Interface (cited:2)

The electronic structure of methanol/TiO₂(110) interface has been studied by photoemission spectroscopy. The pronounced resonance which appears at 5.5 eV above the Fermi level in two-photon photoemission spectroscopy (2PPE) is associated with the photocatalyzed dissociation of methanol at vefold coordinated Ti sites (Ti_5c) on TiO₂(110) surface [Chemical Science 1, 575 (2010)]. To check whether this resonance signal arises from initial or intermediate states, photon energy dependent 2PPE and comparison between one-photon photoemission spectroscopy and 2PPE have been performed. Both results consistently suggest the resonance signal originates from the initially unoccupied intermediate states, i.e., excited states. Dispersion measurements suggest the excited state is localized. Time-resolved studies show the lifetime of the excited state is 24 fs. This work presents comprehensive characterization of the excited states on methanol/TiO₂(110) interface, and provides elaborate experimental data for the development of theoretical methods in reproducing the excited states on TiO₂ surfaces and interfaces.     

Computational Study on Interactions between CO2 and (TiO2)n Clusters at Specific Sites

The energetic pathways of adsorption and activation of carbon dioxide (CO₂) on low-lying compact (TiO₂)_n clusters are systematically investigated by using electronic structure calculations based on density-functional theory (DFT). Our calculated results show that CO₂ is adsorbed preferably on the bridge O atom of the clusters, forming a "chemisorption" carbonate complex, while the CO is adsorbed preferably to the Ti atom of terminal Ti-O. The computed carbonate vibrational frequency values are in good agreement with the results obtained experimentally, which suggests that CO₂ in the complex is distorted slightly from its undeviating linear configuration. In addition, the analyses of electronic parameters, electronic density, ionization potential, HOMO-LUMO gap, and density of states (DOS) confirm the charge transfer and interaction between CO₂ and the cluster. From the predicted energy profiles, CO₂ can be easily adsorbed and activated, while the activation of CO₂ on (TiO₂)_n clusters are structure-dependent and energetically more favorable than that on the bulk TiO₂. Overall, this study critically highlights how the small (TiO₂)_n clusters can influence the CO₂ adsorption and activation which are the critical steps for CO₂ reduction the surface of a catalyst and subsequent conversion into industrially relevant chemicals and fuels.     

Density Functional Characterization of the Electronic Structure and Visible‐Light Absorption of Cr‐Doped Anatase TiO2

To evaluate the electronic and optical properties of Cr‐doped anatase TiO₂, three possible Cr‐doped TiO₂ models, including Cr at a Ti site (model I), Cr at a Ti site with an oxygen vacancy compensation (model II), and an interstitial Cr site (model III), are studied by means of first principles density functional theory calculations. In model I, the splitting behavior of the Cr 3d states and the insulating properties are successfully depicted by the GGA+U method, from which it is proposed that Cr at a Ti site should exist as Cr⁴⁺ instead of the generally believed Cr³⁺. As a result, the electron transitions between these impurity states, the conduction band (CB), and the valence band (VB), as well as the d–d transitions between occupied and unoccupied Cr 3d states, provide a reasonable explanation for the experimentally observed major and minor absorption bands. In models II and III, the impurity states and associated optical transition processes—as well as the corresponding electron configurations—are examined.     

Impact of Oxygen Vacancy on Band Structure Engineering of n-p Codoped Anatase TiO2

Doping with various impurities is an effective approach to improve the photoelectrochemical properties of TiO₂. Here, we explore the effect of oxygen vacancy on geometric and electronic properties of compensated (i.e. V-N and Cr-C) and non-compensated (i.e. V-C and Cr-N) codoped anatase TiO₂ by performing extensive density functional theory calculations. Theoretical results show that oxygen vacancy prefers to the neighboring site of metal dopant (i.e. V or Cr atom). After introduction of oxygen vacancy, the unoccupied impurity bands located within band gap of these codoped TiO₂ will be filled with electrons, and the position of conduction band offset does not change obviously, which result in the reduction of photoinduced carrier recombination and the good performance for hydrogen production via water splitting. Moreover, we find that oxygen vacancy is easily introduced in V-N codoped TiO₂ under O-poor condition. These theoretical insights are helpful for designing codoped TiO₂ with high photoelectrochemical performance.     

Semiempirical computational study of oxygen vacancies in a decahedral anatase nanoparticle

Formation of oxygen vacancies (V_O) is an important step of many catalytic reactions following the Mars van Krevelen mechanism. High rate of oxidation is associated with low energy of V_O formation while high selectivity requires an optimal energy of V_O formation. In the present computational study, enthalpy of V_O formation (ΔH_OVF) is studied in a decahedral anatase nanoparticle (TiO₂)₁₂₁(H₂O)₆ using PM6 method. ΔH_OVF shows large variations for oxygen atoms in different locations on facets, edges and vertices. V_O are much more stable in the (101) facet compared to the (001) facet, while internal V_O are more stable for (101) but equally stable for (001) facet compared to surface vacancies on average. Comparison with literature DFT methods results reveals good consistency and high computational efficiency of the PM6 method for vacancies formation energy. Pm6 also correctly predicts admixture states of the Ti³⁺ within the band gap, but absolute values of electronic band gap and position of admixture states is overestimated and needs scaling factors.     

Evaluation of the structural,optical and photocatalytic properties of nitrogen-fluorine co-doped TiO<Subscript>2</Subscript> thin films

Nanostructured TiO₂ films were synthesized by a modified sol–gel method using Pluronics P-123 (EO₂₀PO₇₀EO₂₀) as templating agent, titanium n-butoxide [Ti(OC₄H₉)₄] as inorganic precursor, and ammonium fluoride (NH₄F) as the source of N and F dopant atoms in order to prepare sols of x(NF) TiO₂ (x=2, 5, 10, 20 wt%). The thin film preparation was made by spin coating, followed by calcination at 400 °C. The as-prepared TiO₂ and xNF-TiO₂ films were characterized by XRD, Raman spectroscopy, FTIR, TGA-DTA, SEM, UV–Vis diffuse reflectance spectroscopy and EPR. XRD and Raman spectroscopy show that the crystalline structure of these samples consists exclusively of the anatase phase. The band gap (E _g ) values for the doped 10 and 20 NF-TiO₂ film systems were found to be significantly smaller than those corresponding to the rest of the other TiO₂ films. The photocatalytic properties of these films are investigated by following the degradation of methyl orange in aqueous solution under UV irradiation. The photodecomposition is mainly a direct function of the amount of NF present in the TiO₂ matrix. The 20NF-TiO₂ sample shows the highest activity of all the samples studied.     

Understanding the synergistic effects,optical and electronic properties of ternary Fe/C/S‐doped TiO2 anatase within the DFT + U approach

Although TiO₂ is an efficient photocatalyst, its large band gap limits its photocatalytic activity only to the ultraviolet region. An experimentally synthesized ternary Fe/C/S‐doped TiO₂ anatase showed improved visible light photocatalytic activity. However, a theoretical study of the underlying mechanism of the enhanced photocatalytic activity and the interaction of ternary Fe/C/S‐doped TiO₂ has not yet been investigated. In this study, the defect formation energy, electronic structure and optical property of TiO₂ doped with Fe, C, and S are investigated in detail using the density functional theory + U method. The calculated band gap (3.21 eV) of TiO₂ anatase agree well with the experimental band gap (3.20 eV). The defect formation energy shows that the co‐ and ternary‐doped systems are thermodynamically favorable under oxygen‐rich condition. Compared to the undoped TiO₂, the absorption edge of the mono‐, co‐, and ternary‐doped TiO₂ is significantly enhanced in the visible light region. We have shown that ternary doping with C, S, and Fe induces a clean band structure without any impurity states. Moreover, the ternary Fe/C/S‐doped TiO₂ exhibit an enhanced photocatalytic activity, a smaller band gap and negative formation energy compared to the mono‐ and co‐doped systems. Moreover, the band edges of Fe/C/S‐doped TiO₂ align well with the redox potentials of water, which shows that the ternary Fe/C/S‐doped TiO₂ is promising photocatalysts to split water into hydrogen and oxygen. These findings rationalize the available experimental results and can assist the design of TiO₂‐based photocatalyst materials.     

Theoretical analysis on TiO2(110)/V surface

Theoretical analysis based on the Hartree–Fock method were performed in order to study the stoichiometric TiO₂ (110) surface and the vanadium substituted system. The Pople with polarization 3‐21G* basis set level was used. The TiO₂ (110) surface was modeled using a (TiO₂)₁₅ cluster model. In order to take into account the finite size of the cluster, we have studied two different models: the point charge and the hydrogen saturated methodologies. The charge values used in the point charge calculations were optimized. The density of states, orbital self‐consistend field (SCF) energies, and Mulliken charge values were analyzed. The method and model's dependence on the analyzed results are discussed. The theoretical results are compared with available experimental data. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

O deficiency in the rutile TiO2 (110) surface: Ab initio quantum chemical investigation of the electronic properties

The (110) surface of rutile TiO₂ (110) has been modeled using a density functional theory (DFT) plane‐wave pseudo‐potential method (CASTEP). In this study, 6 and 9 atomic‐layer slabs have been examined. The stoichiometric surface converges to a low‐spin solution in both cases with a density of states (DOS) similar to that for the bulk. O deficiencies are introduced by the removal of neutral O atoms thus leaving a neutral model with a surfeit of 2 e^? per vacancy. This results in the partial filling of the Ti t_2g conduction band orbitals and a compensatory shift in the Fermi level. The reduced surface converges to a high‐spin solution in all cases, with the excess spin located within the previously unoccupied Ti t_2g orbitals. Removal of the bridging surface O atoms results in an excess spin of 2 electrons per unit cell with approximately one‐half that for removal of in‐plane surface O atoms and subsurface O atoms. The removal of O atoms from the surface leads to an increase of the band gap, with the largest increase due to the removal of in‐plane 3‐fold coordinated surface O atoms, and the smallest one due to the removal of subsurface O atoms. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Systematic study of the effect of HSE functional internal parameters on the electronic structure and band gap of a representative set of metal oxides

The effect of the amount of Hartree–Fock mixing parameter (α ) and of the screening parameter (w ) defining the range separated HSE type hybrid functional is systematically studied for a series of seven metal oxides: TiO₂, ZrO₂, CuO₂, ZnO, MgO, SnO₂, and SrTiO₃. First, reliable band gap values were determined by comparing the optimal α reproducing the experiment with the inverse of the experimental dielectric constant. Then, the effect of the w in the HSE functional on the calculated band gap was explored in detail. Results evidence the existence of a virtually infinite number of combinations of the two parameters which are able to reproduce the experimental band gap, without a unique pair able to describe the full studied set of materials. Nevertheless, the results point out the possibility of describing the electronic structure of these materials through a functional including a screened HF exchange and an appropriate correlation contribution. © 2017 Wiley Periodicals, Inc.     

Insights into the role of D-A-π-A type pro-aromatic organic dyes with thieno[3,4-b]pyrazine as A acceptor group into dye-sensitized solar-cells. A TD-DFT/periodic DFT study

Time-dependent density functional theory (TD-DFT)/periodic DFT calculations were performed to determine the role of pro-aromatic organic D-A -π -A type dyes (the NL1-NL17 family) with Thieno[3,4-b]pyrazine (Tpy) as A acceptor group into dye-sensitized solar-cells (DSSC). This work presents a discussion of the ground and excited states of these dyes along with the aromaticity analysis and the electron injection step using a dye@(TiO₂)₇₂ model. The results suggest that the pro-aromatic behavior increases from the thiophene ring to the pyrazine when an acceptor π-bridge such as phenyl is used. This strong pro-aromaticity is also reflected in the electron injection step, studied using a 3x2 3 layer (TiO₂)₇₂ slab model. The resulting adsorption energies (ΔE_ads and ΔG_ads) and the electron injection (ΔG_inject) in the stablest coordination mode, Bid_CN_COOH, indicate that the redox reaction (Dye* ➔ Dye⁺ + e⁻) is stronger and more spon than the adsorption reaction (Dye⁺ + TiO₂ [+e⁻] ➔ Dye@TiO₂) in the electron injection. In this way, the highest efficiency of NL6 and NL12 is a consequence of the more significant pro-aromatic characteristics and the more spontaneous redox process. Finally, these NL dyes are promising in the molecular engineering of D-A -π -A metal-free types dyes.     

Surface structure for Au on (0 0 1) SrTiO3

First-principle calculations are performed to study the crystal structure, formation energies, and electronic structures of (0 0 1) SrTiO₃ surfaces with/without Au covered. The initiative Au additive layer is crystallized in a fcc structure with (0 0 1) face on SrO-terminated surface. The bimodal growth trend of Au on TiO₂-terminated surface is qualitatively consistent with the experimental observations. The defect structure of Au occupying the oxygen (O) vacancies of TiO₂-terminated surface is energetically favorable under oxygen-poor conditions, and a feature corresponding to gap states appear and the occupied Ti 3d states disappear.     

Semiempirical calculations of TiO2 (rutile) clusters

The densities of states for small (TiO₂)_x-clusters, x = 1, 3, 6, 9, and 14, have been calculated by means of the INDO method. The shape of the valence bands' density of states (DOS ) are discussed in terms of the distribution of coordination numbers. A one-slab cluster with uniform distribution of the coordination numbers was used to compare our calculations with experimental spectra. The photoelectric DOS and DOS for a cluster with an oxygen vacancy are in very good agreement with experimental findings for the TiO₂ (001) surface. O1s core level shifts between a surfacelike and a bulklike oxygen atom have been estimated. It is concluded that the observed surface–bulk shift for the TiO₂ (001) surface contains a substantial relaxation contribution. © 1992 John Wiley & Sons, Inc.     

Controllable Synthesis of Inverse Opal TiO2‐x Photonic Crystals and Their Photoelectric Properties

In this study, inverse opal TiO_2‐x photonic crystals (IO‐TiO_2‐x) have been successfully synthesized by a two‐step calcination. The whole synthesis is safe and feasible. Additionly, the reduction degree and the structure of IO‐TiO_2‐x can be precisely controlled. A series of IO‐TiO_2‐x samples with different reduction degree were prepared and characterized. The TEM images show that the obtained samples possess a 3D‐ordered macroporous inverse opal structure. The reduced Ti atoms/oxygen vacancies were confirmed by Raman and XPS spectroscopy. All IO‐TiO_2‐x samples showed better photoelectric properties than those of common TiO₂ which indicates their great potential to be applied to photoelectric fields. The improvement of photoelectric properties is attributed to the efficient electron‐hole separation efficiency induced by moderately reduced Ti atoms/oxygen vacancies. Meanwhile, the 3D‐ordered macroporous inverse opal structure and the band gap are regulated to “capture” more solar energy. This new approach is proven to be a meaningful method to synthesize high‐performance TiO₂ materials.     

B离子掺杂TiO2催化剂(TiO2－xBx)光催化活性的研究

采用溶胶-凝胶法制备出纳米TiO₂和TiO_2－xB_x催化剂. 光催化实验证明, TiO_2－xB_x催化剂的紫外、可见光催化活性均高于TiO₂. XRD, XPS和Raman结果表明, B离子是以取代式掺杂占据了TiO₂的O^2－的晶格位置. UV-Vis和PL谱的结果表明, B离子的2p轨道与O的2p轨道形成混合价带, 产生可见光响应, B离子的掺入有效地阻止了光生载流子的复合, 促进了其分离, 是TiO_2－xB_x催化剂紫外、可见光催化活性提高的主要原因.     

Theoretical Characterisation of Phosphinyl Radicals and Their Magnetic Properties: g Matrix

The g matrices (g tensors) of various phosphinyl radicals (R₂P^.) were calculated using the DFT and multireference configuration interaction (MRCI) methods. The g matrices were distinctly dependent on the molecular structure of the radical. To thoroughly examine this dependence, the contributions from individual atoms and excited states were calculated. The former revealed the gain from the phosphorus atom to be preeminent unless P?O or P?S bonds are present in the radical molecule. The contributions owing to excited states arising from electronic transitions between doubly occupied molecular orbitals and the SOMO were clearly positive, as in the case of semiquinone and niroxide radicals. The transitions from the phosphorus lone pair were of paramount importance. Surprisingly, unlike for semiquinones and nitroxides, a significant negative contribution was observed from excitations from the SOMO to unoccupied molecular orbitals. For radicals with P?O bonds, this contribution to the g₂ component was dominant.     

Preparation and characterization of nitrogen-doped TiO2 photocatalyst in different acid environments

Nitrogen-doped TiO₂ powders were successfully prepared by a wet method, i.e., a micro-emulsion-hydrothermal method, in different acid environments. Several characterization techniques, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV-visible diffuse reflectance spectra, were combined to determine the crystal phase, concentration and chemical states of the nitrogen doped in TiO₂. The high photocatalytic activity of the nitrogen-doped TiO₂ was evaluated through the decomposition of rhodanmine B under visible light irradiation. It was suggested that the doped nitrogen formed oxynitride (NO) and produced impurity states at higher above the valence band of TiO₂. Therefore, the nitrogen doping could enhance the response of photocatalyst to the visible light and improve the photocatalytic activity because of the narrowing of band gap of TiO₂.